Electrodeposition of silver and its alloys has been practiced for many years, particularly in the decorative fields. The electrodeposition of silver has also been fairly widely used in the electrical and electronics industries, in addition to the electrodeposition of gold and as a replacement for gold.
One of the main problems involved in the electrodeposition of silver is that most metal substrates desired to be plated with silver will become plated with silver by immersion. This silver immersion plating occurs when no potential at all is being applied. This immersion plate is poorly adhered to the substrate resulting in poorly adhered electrodeposits. Thus, in electroplating silver it is common practice to first deposit a very thin layer of silver onto the substrate by what is known as silver strike. These silver strike solutions generally have low silver content and a high free cyanide content. With such solutions containing low silver and high free cyanide content, immersion plating of the substrate, such as a copper substrate, is sufficiently minimized or sufficiently adhered to the substrate to permit silver deposits to be electroplated thereon as adequately adherent deposits. The substrate having the thin strike layer of silver deposited thereon can then be electroplated using a conventional silver electroplating bath containing a higher silver content. Since silver will not immersion plate onto silver, a second and thicker coating of silver can be applied to the strike layer with satisfactory or good adhesion.
The problem of silver immersion taking place on substrates is even more pronounced when attempting to electroplate silver with the spot-plating process or when utilizing high-speed silver plating baths. Although the selective plating machines presently in use differ in many mechanical aspects, the silver plating cells found therein are essentially similar in operation in the respect that the plating solution is sprayed out at high velocity from jets or nozzles or similar type orifices against the plating surface. This results in a vigorous agitation in the confined cathode space, as is normally desired, and results in the high-speed plating of pure silver onto a limited plating area. In this process it is not only necessary to utilize a silver strike solution prior to the final electrodeposition of silver, but a further problem is encountered in that the silver immersion deposit takes place onto the metal base substrate strip during the off cycle of the spot-plating process. For example, in utilizing a device such as disclosed in the Johnson et al. U.S. Pat. No. 3,723,283, a spot silver strike would be applied at station S9 and the silver electroplated at station S11. At station S11 (see FIG. 4) the electrolyte is continuously pumped to the header. When the header is in the "on" position, the strip 10 is appropriately masked, the current is on by contact with the electrode, and the unmasked portions of the strip are electroplated. When the strip 10 is moved to electroplate the next area, the header is released, unmasking the entire strip, and the current is off. The electrolyte, however, is still being splashed onto the strip and causes silver immersion over the entire unmasked strip. Such silver immersion deposit not only interferes with the electrical properties desired, but it obviously results in a loss of a valuable metal.
Generally, silver cyanide electroplating baths, even when the substrate has been previously strike plated, deposit a dull, non-lustrous white layer of silver on the substrate requiring buffing to secure a bright deposit. This problem has been overcome to a significant extent in the prior art by adding brightening agents to the silver cyanide plating baths. Many brightening agents have been proposed for these baths including tellurium, selenium and sulfur compounds. The sulfur compounds are the ones that are predominantly used commercially today. Thus, the addition of the sulfur compounds to the conventional silver cyanide baths containing a high content of free cyanide act as brightening agents and generally result in the deposition of a highly lustrous, silver deposit needing little or no buffing.
These baths to which the sulfur compounds have been added, however, still deposit silver by immersion onto the metal substrate and therefore silver strike baths are still necessary to insure adequate adhesion of the electroplated silver. Examples of conventional silver baths and sulfur compound brightening agents are set forth in U.S. Pat. Nos. 2,110,792; 2,113,517; 2,800,439; 3,362,895; 3,446,716; and 3,580,821, as well as in an article entitled ELECTRODEPOSITED SILVER AND ITS ALLOYS, ELECTROPLATING AND METAL FINISHING, pp. 3-5 and 8-13 (June, 1976).
High-speed silver electroplating baths of the cyanide type containing little if any free cyanide, also have the disadvantage of silver immersion plating on the substrate and thus a silver strike is still preferred to secure the desired adhesion of the final silver deposit.
Although many non-cyanide silver plating baths are known and have been suggested for use in low-speed as well as high-speed plating, none of these have reached the stage of commercial significance. A thiosulfate silver plating bath is one example, but its use has been limited since it also requires a silver strike from a different type of bath, namely with a silver cyanide bath high in free cyanide and low in silver content. Attempts to use a thiosulfate strike bath, high in free thiosulfate and low in silver content, have not been successful since these baths also deposit silver by immersion and therefore interfere with the silver adhesion deposited by the main silver thiosulfate baths.